Unmanned aircraft or air vehicles (UAVs) provide enhanced and economical access to areas where manned flight operations are unacceptably costly and/or dangerous. For example, unmanned aircraft outfitted with remotely operated movable cameras can perform a wide variety of surveillance missions, including spotting schools of fish for the fisheries industry, monitoring weather conditions, providing border patrols for national governments, and providing military surveillance before, during, and/or after military operations.
Many unmanned aircraft systems (which can include the aircraft itself along with launch devices and recovery devices), however, can be difficult to install and operate in cramped quarters, such as the deck of a small fishing boat, land vehicle, or other craft. Accordingly, the operation of such aircraft systems often includes retrieval or capture of the aircraft with a vertically oriented recovery line when space is insufficient for a normal landing run. One concern with capturing aircraft using vertically oriented recovery lines, however, is that tension in the line must be precisely controlled to avoid damaging the aircraft and/or the support structure from which the recovery line is suspended during capture and post-capture operations. For example, if the recovery line is not tight enough, the line may not sufficiently impede the aircraft's motion after capture, which can result in the aircraft receiving a hard stop or jerk as the recovery line tightens after capture. On the other hand, if the recovery line is too tight, the aircraft can bounce off the line and not be captured at all. In either case, the aircraft and/or the support structure can be damaged.
One conventional method for measuring the tension in the recovery line is the “finger” test in which an operator feels the recovery line with his or her fingers to see if the tension “feels right.” One drawback with this method is that it is completely operator-dependent, and the measurements can vary widely from person to person. Another conventional method for measuring the tension in the recovery line is to attach a “fish scale” or other type of force scale to the recovery line and pull back on the scale to measure the tension. One drawback with this approach, however, is that the readings from the scale can vary significantly depending on where on the recovery line the scale is positioned and the amount of force the operator applies when pulling back on the scale. Furthermore, measurements from the scale are often interpreted very differently by different operators, which can result in inconsistent and/or inaccurate measurements.